In this document, the following abbreviations are used in the descriptions of both the prior art and the invention:    CoS Class of Service,    DRR Deficit Round Robin, a scheduling method based on a weighting factor [2],    DT Data Transfer, a service-quality class for applications, for which upper-limit values are not guaranteed for data-transmission delay and variation in delay,    DSCP Differentiated Services Code Point, information carried by a packet, as to which service-quality class the packet in question belongs,    MDRR Modified Direct Round Robin, a scheduling method based on a weighting factor [3],    QoS Quality of Service,    RT Real Time, a service-quality class for applications, for which an attempt is made to minimize the data transmission delay and the variation in delay, but which do not benefit from momentarily free telecommunications network capacity,    SFQ Start-time Fair Queuing, a scheduling method based on a weighting factor [1],    wfq weighted fair queuing, a scheduling method based on a weighting factor, used as a general name,    WFQ Weighted Fair Queuing, a scheduling method based on a weighting factor [1].
In a packet-switching telecommunications system, it is often advantageous for the telecommunications packets (Pkt, FIG. 1, hereinafter ‘packet’) to be classified as belonging to different service-quality classes (CoS), according to the requirements of the application using the telecommunications service, and, on the other hand, to the kind of agreement on the quality of service that has been made between the telecommunications service provider and its customers. For example, in the case of a normal telephone application, it is essential that the data-transmission speed required by the application is available for the time needed and that the transmission delay is sufficiently small and the variation in the transmission delay is sufficiently low. In a telephone application, there is no benefit in the data transmission speed provided for the application being able to be increased momentarily, if the loading of the telecommunications network is low at the time in question. On the other hand, for example, when downloading a www page, it is extremely advantageous to be able to exploit to the full the free capacity of the network even momentarily.
By way of example, let us examine a situation, in which a telecommunications service offers the following kind of service-quality classes:                RT (Real Time): for applications, for which the data transmission delay and the variation in the transmission delay are attempted to be minimized, but the momentary data transmission speed provided for the application is not increased, even if the loading of the telecommunications system at the time in question is low,        DT (Data Transfer): for applications, for which upper-limit values are not guaranteed for the transmission delay and the variation in the delay, but for which the free capacity of the telecommunications system at each moment in time is exploited.        
Let us examine, by way of example, the following situation in FIG. 1, in which the capacity of a telecommunications link S is divided between two customers A1 and A2, in such a way that both customers are given at least 50% of the capacity. If one of the customers uses less capacity than their own 50% quota during a specific period of time, the free capacity during the period in question is available to the other customer. Both customers can produce both RT and DT traffic. Because the RT traffic is delay-critical, the RT traffic is assumed to be limited so that the customer can produce RT traffic to a maximum of 50% of the telecommunications link S. If the limitation in question were not imposed, the buffer memories of the network element NE1 could become filled uncontrollably, in which case the transmission delays would also grow uncontrollably. In order to limit the delays in the RT traffic, the packets representing the RT traffic are scheduled to the transmission link S with a higher priority than that of packets representing DT traffic.
FIG. 2 shows one way according to the prior art for scheduling the capacity of a common transmission link in a situation like that described above. The operation of the system shown in FIG. 2 is as follows:                the service-quality class to which an individual packet belongs can be identified on the basis of information attached to the packet (for example, DSCP=Differentiated Services Code Point[2]),        the packets are directed to service-quality class and customer-specific buffer memories (customer-specific RT and DT buffers),        in the internal processing of the customer's traffic, the packets representing RT traffic are scheduled with a higher priority than that of the packets representing DT traffic,        each customer's RT-class traffic is assumed to be limited (≦50% of the capacity of the transmission link) before the scheduler apparatus.        
A problem in the method shown in FIG. 2 is that a traditional scheduling method based on a weighting factor (wfq) processes the traffic components (L1 and L2) offered to the scheduler in such a way that both receive the share of the transmission link S capacity (in this case, 50% for each). If, for example, an RT packet is offered from customer A1 and a DT packet is offered from customer A2, there is no way to guarantee that the RT packet will be scheduled to the transmission link before the DT packet. In general, the scheduling sequence is random. Because RT packets represent delay-critical traffic, they should, however, always be scheduled to the transmission link S before the DT packets.
FIG. 3 shows another way according to the prior art to schedule the capacity of a common transmission link S in a situation like that described above. In the situation shown in FIG. 3, the scheduling operation is as follows:                the customers' A1 and A2 shares of the traffic RT (RTA1 and RTA2) are scheduled using a weighting-factor based scheduling method (wfq). The DT shares (DTA1 and DTA2) are scheduled in a similar manner. The wfq schedulings of the RT and DT shares are handled independently of each other.        the final selection of the packet to be sent to the transmission link is made between the RT and DT traffic, using a priority-selection method.        
A problem in the scheduling method shown in FIG. 3 is that customer A1 or A2 will not receive their promised 50% share of the capacity of the transmission link S in all circumstances. For example, let us examine a situation, in which customer A1 produces 50% of the RT-traffic capacity of the transmission link S and also DT traffic to 50% of the capacity of the transmission link S. Customer. A2 produces only DT traffic at a speed of 50% of the capacity of the transmission link S. In this situation, the desired, operation would be for the RT traffic produced by customer A1 to proceed and the DT traffic to be dropped, while the DT traffic produced by customer A2 would be allowed to proceed. Thus customer A1 would use its own 50% quota for RT traffic, the transmission of which being regarded as more important than the transmission of the DT traffic, while customer A2 would use its own 50% quota for DT traffic. How to divide their own capacity shares between RT and DT traffic would remain the customers' own internal matter. However, in the system according to FIG. 3, the capacity of the transmission link S is divided as follows: 50% goes to the RT traffic of customer A1 while the remainder is divided equally between the DT-traffic shares of customers A1 and A2. This means that customer A1 gets 75% and customer A2 25% of the capacity of the transmission link S. The situation is not satisfactory, because customer A2 does not get the 50% share of the capacity of the transmission link S that it has been promised.